1. Field of the Invention
The present invention relates to a display panel employing an organic electroluminescence (hereinafter referred to as OLED: Organic Light Emitting Diode) and a production method thereof, in particular to an OLED display panel having an improved light extraction efficiency and display contrast.
2. Description of Related Art
In an OLED display employing an OLED display panel, self-luminous OLED elements are arranged on a substrate, such as a glass substrate to show information by emitting the OLED elements. The OLED display is advantageously thin and light in weight, high in image quality, superior in dynamic image display, wide in the angle of view, and low in power consumption as compared with other types of flat-type displays. The OLED display, therefore, is considered an epoch-making flat-type display supporting the ubiquitous society.
OLED elements are generally configured by sandwiching at least one organic layer between an anode and a cathode. At least one organic layer herein described has a different structure and number of layers according to its element structure, but in many cases, the organic layer includes three to five layers of functional layers, such as a hole-transporting layer, a hole-injecting layer and/or an electron-transporting layer, and an electron-injecting layer or the like, which sandwich a light-emitting layer.
A hole is fed from the anode into a light-emitting layer and an electron is fed from the cathode into the light-emitting layer through the above-mentioned each functional layer by the application of a direct current voltage between two electrodes: the anode and the cathode. The electronic state of organic molecules included in the light-emitting layer is changed to the excited state by energy generated by a recombination of the hole and the electron in the light-emitting layer. Energy is emitted as light when this quite unstable electronic state falls to a ground state to emit organic light emitting diodes. Organic electroluminescence is referred to also as organic light emitting diode (OLED) because this emitting principle is common to the emitting mechanism of light emitting diode (LED).
A method of taking out luminance of an OLED display has two systems: bottom-emitting system and top-emitting system. As shown in FIG. 7(a), the bottom-emitting system takes out light from an insulating substrate side 2010. As shown in FIG. 7(b), the top-emitting system takes out light from a top surface layer side 1014.
Conventionally, for example, as shown in FIGS. 4(a) to 4(e), a top-emitting OLED display panel is manufactured as follows: (1) as shown in FIG. 4(a), a substrate 110 is prepared to deposit an anode layer 115 by sputtering or evaporation or the like. (2) As shown in FIG. 4(b), the anode layer 115 is patterned by a photolithographic method to form anode layers 114 for each pixel area by removing unnecessary part by etching. (3) As shown in FIG. 4(c), edge insulators 113 are formed between each anode layer 114 by patterning using the photolithographic method after forming a film by a spin coating method or the like. (4) As shown in FIG. 4(d), walls 118 are formed on the edge insulators 113 by patterning using the lithographic method after forming a film using the spin coating method or the like. (5) As shown in FIG. 4(e), organic layers 120 are formed using metal masks in a vacuum deposition method while performing patterning, and then cathodes are formed using a resistance heating method, an electron beam evaporation, and a sputtering method or the like.
To increase luminance of the OLED display, it becomes necessary to increase a current to be fed or to have a panel with superior efficiency. Improving the light extraction efficiency of the OLED display (amount of light which can be extracted from the light extracting side/total amount of emission from OLED elements) as much as possible is needed to prepare OLED elements with superior efficiency.
The thickness of the organic layer to cause light emission is up to 1,500 angstrom or so, so that as far as the light-emitting layer is concerned, its thickness is only some hundred angstrom or so, which is quite shorter than the emission wavelength. Such light emission caused within the layer spreads to all directions of solid angle Ω=4π within the film. As schematically shown in FIG. 8, generally, in a top-emitting OLED display 1001, light emitted to an anode side 1012 is reflected on the anode 1012 to be emitted from a cathode side 1014 at the same time when light is emitted through the cathode 1014. In the bottom-emitting OLED display, respective roles of the anode 1012 and the cathode 1014 are opposite to those of the top-emitting OLED display.
As mentioned above, since light is emitted within the layer with a sufficient thinness, as shown in FIG. 6, the rate of emission of total reflection becomes extremely high, unless the refractive index in the interface between the emitting layer and the functional layer or between the functional layers is sufficiently increased. Light totally reflected is propagated inside the organic layer 120 so that the light may pass a waveguide to emit in parallel with the interface between the layers. The luminous efficiency against incident energy of such light in parallel with the interface between the layers is deteriorated due to no contributing emission components to the luminance of the display.
As shown in FIG. 5, in the conventional structure of OLED elements, there are several polymer structures around the emitting region of the OLED elements 103, such as edge insulators 113 to prevent the anodes 114 and the cathodes 122 being short, and walls 118 to divide the cathodes 122 at a certain position. Although there are some colored ones among these polymer structures, many of them do not have a sufficient optical density (OD) to block light. Accordingly, these polymer structures function like an optical waveguide by incoming light that is in parallel with the above-mentioned interface between the layers, which results in the passing of light.
Such light does not greatly contribute to such contrast that is evaluated by the OLED display panel in total black versus total white. However, there were problems with checker patterns, such as ANSI contrast and the damage of effective contrast ratio that is difficult to evaluate by the figure at the time of showing ordinary image.
Cited document 1 discloses an invention having a mesa structure in which OLED elements are sandwiched between inverted V-like structures having a refractive index higher than the emitting layer. According to the invention described in the cited document 1, an image forming apparatus which is excellent in visibility and which can maintain a light emission performance with a high degree of efficiency can be provided.
(Cited Document 1)
Japanese Publication No. 2003-257659 (Paragraphs 53 to 56)
When compared to a conventional OLED display panel in which all structures are formed of polymer, the mesa structure has, however, various restrictions to materials used for an inverted V-like structure and a method for forming an OLED element, which results in complicated production processes and high production costs.
Although examples of materials having light reflectivity for the inverted V-like structure include metal materials and conducting materials, this structure has difficulty in effectively preparing an OLED panel without any modifications due to defects, such as a short of the anode and the cathode around pixels. Further, the materials may be optically transparent, but they have a problem with little effects of improving the above-mentioned effective contrast ratio.
Thus, it is an object of the present invention to provide an OLED display panel having an electrode structure capable of forming peripheral structures, such as walls by polymer as well as simultaneously improving the brightness and the contrast because of preventing an emission component which has been conventionally useless parallel to an electrode surface from intruding into a polymer structure by releasing the parallel emission component in a direction vertical to the electrode surface.
An OLED display panel according to the present invention comprises: a substrate; a plurality of banks, each having a tapered shape of cross section comprised by a pair of side surfaces, an upper surface and a bottom surface respectively connected to the side surfaces, the banks being arranged on a top surface of the substrate in side by side at an interval; and an electrode layer covering the top surface of the substrate between banks and the side surfaces of the banks adjacent to such top surface. The banks may be formed of polymer. And an angle formed by the bottom surface and one of the side surfaces is preferably not less than 40° degrees and not more than 50°. The insulating film is preferably formed of polymer, such as acrylic polymer and polyimide polymer having a visible light transmittance of not less than 95%.